FIG. 7 is a schematic view showing a conventional terminal structure for cryogenic equipment.
This terminal structure has a terminal of cryogenic equipment 100 (not shown), a refrigerant tank 10 for receiving the terminal, a bushing 30 for securing electric conduction from a conductor of the vacuum vessel 20 to a room temperature portion, a vacuum vessel 20 covering the outside of the refrigerant tank 10, and an insulator 40 connected to an upper portion of the vacuum vessel 20 so as to protrude therefrom.
The bushing 30 is connected, substantially at right angles, to a superconductor led in from the cryogenic equipment 100. The bushing 30 has a conductor in its core, and is circumferentially coated with solid insulation such as ethylene-propylene rubber or the like. The bushing 30 penetrates the joint surface between the vacuum vessel 20 and the insulator 40 so as to be received in the insulator 40. The inside of the insulator 40 is filled with insulating fluid 43 such as insulating oil, sulfur hexafluoride SF6, or the like.
Liquid nitrogen 11 supplied from a supply pipe 22 is accumulated in the refrigerant tank, while a nitrogen gas reservoir 15 is formed above the liquid nitrogen 11. This nitrogen gas can be discharged through a gas discharge port 14.
Thus, in such a terminal structure, the conduction portion from the cryogenic equipment 100 to the insulator 40 passes the very low temperature portion immersed in the liquid nitrogen 11, the nitrogen gas reservoir 15 and the room temperature portion in the insulator, in the order of increasing distance from a cable 80.
However, the aforementioned terminal structure has the following problems.
(1) Thermal invasion from the room temperature portion 400 to the very low temperature portion 200 is large.
When the nitrogen gas reservoir 15 exists between the liquid level of the liquid nitrogen 11 and the upper surface of the vacuum vessel 20, heat conduction occurs from the vacuum vessel 20 to the room temperature portion 400 due to the convection of the nitrogen gas. The temperature of the liquid nitrogen increases in accordance with the heat conduction. As a countermeasure against the temperature increase, cooling is carried out correspondingly to the temperature increase. As a result, the energy required for the cooling becomes a loss to thereby cause increase in the system loss as a whole.
(2) Application of regenerative cooling using a closed system to the refrigerant tank is difficult.
A part of the bushing 30 is usually immersed in a refrigerant. The liquid level of the liquid nitrogen 11 is controlled so as to be maintained at a level required for this immersion. When the refrigerant tank is an open system, the liquid level can be maintained by supplying the liquid nitrogen 11. However, when regenerative cooling is carried out with the supply pipe 22 and the discharge port 14 being closed to leave the liquid nitrogen 11 as it is, a change in pressure, a change in invading heat, and so on, make control of the liquid level be difficult.
On the other hand, the bushing 30 is connected, substantially at right angles, to the connecting cable (conductor) 80 connected to the superconductor of the cryogenic equipment 100. The bushing 30 is, for example, formed by inserting a conductor of copper or the like into the core of a stainless steel pipe, and coating the circumference of the stainless steel pipe with solid insulation such as ethylene-propylene rubber or the like. One end of the bushing 30 is immersed in the refrigerant while the other end thereof penetrates the joint surface between the vacuum vessel 20 and the insulator 40 so as to be received in the insulator 40. The inside of the insulator 40 is filled with insulating fluid 43 such as insulating oil, SF6 or the like. When the insulator 40 is filled with insulating oil, a gas reservoir may be formed above the insulating oil. In the inside of the stainless steel pipe, there is a space connecting very low temperature with room temperature. This space may communicate with the aforementioned gas reservoir or may not communicate with the aforementioned gas reservoir.
The liquid nitrogen 11 supplied from the supply pipe 22 is accumulated in the refrigerant tank 10, and a nitrogen gas reservoir portion 13 is formed above the liquid nitrogen 11. This nitrogen gas can be discharged through the gas discharge port 14.
Thus, in such a terminal structure, the conduction portion from the cryogenic equipment 100 to the insulator 40 passes the very low temperature portion 200 immersed in the liquid nitrogen 11, the nitrogen gas reservoir portion 13, and the room temperature portion 400 in the insulator 40, in the order of increasing distance from the cable.
However, the aforementioned terminal structure further has the following problems.
(3) When the space inside the bushing 30 communicates with the gas reservoir 42 in the insulator 40, the insulation performance may deteriorate.
If the air exists in the space which is formed in the housing 30 for connecting the very low temperature to the room temperature, liquefaction and freeze occurs in the air due to the very low temperature so that the volume of the air is reduced greatly. As a result, the inside of the insulator 40 becomes negative pressure, resulting in deterioration of insulation performance.
It can be also considered that a gas supply unit is connected so that gas supply can follow the volume change of gas in the space of the bushing 30. However, since a gas inlet 22 becomes a high voltage portion, shutdown of current application and attachment/detachment of pipe arrangement are required for the connection of the gas supply unit in an unrealistic sense.
It can be also considered that gas is not supplied to the space of the bushing 30, but the space in the bushing 30 and the gas reservoir 42 in the insulator 40 are kept at high pressure in advance so as to prevent the insulation performance from being affected by the lowering of pressure caused by cooling. However, in order to secure pressure large enough not to affect the insulation performance after cooling, the pressure before cooling should be excessive in an unrealistic sense.
(4) When the space inside the bushing 30 does not communicate with the gas reservoir 42 in the insulator 40, mechanical damage may be caused by an excessive pressure change in the inside of the bushing.
In the configuration in which the space inside the bushing 30 does not communicate with the gas reservoir 42 in the insulator 40, the space inside the bushing 30 is generally sealed off. Therefore, the aforementioned deterioration of the insulation performance caused by the negative pressure is insignificant. However, it is also anticipated that the sealing of the space inside the bushing 30 is not perfect. In that case, there is a possibility that the space communicates with the gas reservoir 42, though slightly. From a long-term perspective, it can be considered that the gas in the space of the bushing 30 is liquefied to form negative pressure so that the air of the gas reservoir 42 is sucked into the inside of the bushing 30 gradually due to the state of negative pressure. Then, when the air sucked from the gas reservoir into the space inside the bushing 30 is liquefied, very high pressure is formed when the air returns to the room temperature. Thus, the very large pressure causes mechanical damage to the bushing 30.